For the high-voltage source of X-ray generators, a resonant converter in full-bridge (i.e. H-bridge) configuration has turned out to be a useful topology. More generally, the resonant converter in full-bridge configuration is capable of converting a DC current into an AC current or vice versa, and thus it can be used as a power inverter or a controllable rectifier.
FIG. 1 illustrates an exemplary high-voltage source comprising a resonant converter in full-bridge configuration. The full-bridge configuration comprises two parallel switching branches (also called half-bridges or switching legs): one comprises two series-connected switch members S1 and S2, and the other comprises two series-connected switch members S3 and S4. Each switch can be any suitable type of power semiconductor device and is exemplarily illustrated as an IGBT transistor with a reverse conducting diode in FIG. 1. A current is conducted on average between the DC voltage source Vdc_in and the full-bridge. A DC-link capacitor C_in or a set of those capacitors may be used to conduct AC components of the inverter input current in order to smooth the DC voltage across the switching legs. A resonant circuit is connected in series with a transfer former primary (low-voltage) winding of a transformer T between the junctions A and B of the switch members in each of the switching branches. A resonant load circuit, also called resonant tank, can be driven by the full-bridge inverter. For example, it can be a series resonant circuit (i.e. a LC circuit) or an LCC circuit. Other combinations of resonant elements are possible. For example, the capacitor C_res, the inductor L_res, and the inherent parasitic capacitance C_p which arises from the secondary (high-voltage) winding of the transformer T, form the LCC circuit. The switching events of the full-bridge are controlled by 110, 112, 114, 116 of a control circuit (not shown) so as to convert a DC link voltage to an AC voltage V_tank which drives the resonant circuit. The arising AC current is then transformed to a high-voltage level by the transformer T, and is then rectified and smoothed by an output capacitor C_out. The inherent parasitic transformer capacitance C_p creates favorable conditions during current commutation and may be used to boost the output voltage. The output voltage can be supplied to any kind of load L, such as an X-ray tube.
There exist several control schemes for controlling the switching events of the full-bridge. The main objective is to define a scheme which enables a high operating frequency in order to reduce the ripple of the output voltage and to minimize the cost and size of components. This can only be reached if the power semiconductor losses can be minimized by means of zero voltage switching (ZVS) and zero current switching (ZCS). ZVS may be supported by use of snubber capacitors Cs1 . . . Cs4, which are connected in parallel to the switching devices, respectively, as seen in FIG. 1. Furthermore, they help to relieve electromagnetic interference and reduce earth leakage currents.
A well-known control scheme is pulse frequency modulation (PFM). This means that the transferred output power depends on the controlled operating frequency of the inverter. The most often used control method is operation with variable frequency in a range which is located above the series resonant frequency of the main reactive elements L_res and C_res according to the circuitry in FIG. 1. This control generally requires wide frequency variation in order to cover a range of operation from no load to full load. Furthermore, there are operational states in which the power is fed back from the resonant reactances to the DC input capacitor C_in. This power feedback is active when a diagonal pair of diodes, either D1 and D4, or D2 and D3, conduct a current. Consequently, the drawbacks of the existing PFM control technique are larger size and lower efficiency.
Other control schemes, such as pulse width modulation (PWM, see U.S. Pat. No. 5,719,759A), apply both kinds of hard switching events (turn-off and turn-on) to any semiconductor switch, which makes it difficult to use a simple snubber circuit such as a snubber capacitor. Hard switching events cause high levels of switching losses and thus limit the operation frequency according to the heat dissipation capabilities of the semiconductor devices.
Another control scheme called phase shift control (see US2011222651, U.S. Pat. No. 6,178,099B1) uses different modes at different power ranges. The leading leg half-bridge and the lagged leg operate with different switching actions. Under light-load or no-load condition, the ZVS condition is no longer present. At high power, the ZCS condition is difficult to keep. Thus, there are operating points where the phase shift switching method does not work in soft switching. Switching losses become significant there and lead to an undesired cooling effort or even to over temperature of the devices.